(Bloomberg) -- Operations at a Zijin Mining Group Ltd. copper and cobalt mine in the Democratic Republic of Congo have been suspended due to worries about excessive radiation in its cobalt, according to people familiar with the matter.

The La Compagnie Minière de Musonoie project, known as COMMUS, has been closed since earlier this week, the people said, asking not to be identified because they aren’t authorized to speak on the matter. 

Government officials present at COMMUS are overseeing the suspension of production and testing cobalt shipments that were blocked and returned after being exported, the people said. Congo has some deposits of uranium in its southeastern Katanga region and some contamination occurs occasionally.

Cobalt is mined as a by-product of copper production and the COMMUS mine produced 129,000 tons of copper last year, about 4.2% of total output in the world’s second largest source of the metal. It also produced 2,200 tons of cobalt last year.

The shutdown at the project comes during a period of supply disruption in the global copper market, driven by the surprise closure of First Quantum Minerals Ltd.’s giant Cobre Panama mine. Anglo American Plc also downgraded production guidance and Codelco has seen output slump to quarter century lows. 

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China’s Zijin owns 72% of COMMUS and the project is in the mining hub of Kolwezi in Lualaba province. The state-owned Gecamines controls the balance. Zijin also holds an interest in Congo’s largest copper project, Ivanhoe Mines Ltd.’s Kamoa-Kakula.

Zijin, Congo’s Mines Minister Antoinette N’Samba Kalambayi and Lualaba’s Mining Minister Jacques Kaumba didn’t respond to requests for comment. Gecamines declined to comment.

--With assistance from Alfred Cang and Winnie Zhu.

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